Production of metallic powders



United. States a Patent PRODUCTION OF METALLIC POWDERS Peter Robert Marshall, Sheffield, England, assignor to The Birmingham Small ArmsCompany Limited, Birmingham, England No Drawing. Application May 15, 1952,

Serial No. 288,055

1 Claim. (Cl.204--) This invention is concerned with the production of metallic powders and is particularly concerned with the production of stainless steel powder suitable for use in powder metallurgical processes.

Many methods for the production of metallic powders have been proposed and used, and suchmethods include atomising, vapourising or spraying the metal in molten form, or physically contacting the metal in molten form with other fluids, or mechanically comminuting the metal by cutting, rolling orcrushing operations. Chemical methods are also used and may make use of precipitation, reduction or electrolysis.

Insofar as electrolytic methods are concerned, they have, as far as we are aware, been carried out by utilising a salt of the metal to be powdered, the salt sometimes being in a fused condition. For example, iron powders have been producedby electrolyzing ferrous chloride solution in the presence of formic acid. Furthermore, as far as we are aware, these electrolytic methods have only been used for the purpose of providing powders of metallic elements and not powders of metallic alloys.

It is one of the objects of this invention to provide an improved electrolytic method for the production of stainless steel powder. I

In accordance with the invention, a method of producing stainless steel powder includes heat treating stainless steel in massive form to precipitate carbides on the grain boundaries, and immersing the stainless steel so treated as an electrode in an electrolytic bath so as to induce electrolytic intergranular attack in the stainless steel.

According to a feature of the invention, the stainless steel has a carbon content lying within the range 0.1% and 0.27%.

In carrying the invention into effect, according to one form by way of example, in order to produce stainless steel powder suitable for use in powder metallurgical processes, massive stainless steel in bar form is used, being obtained by rolling, casting or any other suitable method. The stainless steel may usefully be of the 18-8 type with a carbon content of 0.17% and should be such that it is free from carbide-stabilising elements, examples of which are titanium, niobium, tantalum, tungsten and molybdenum.

The stainless steel bars are initially treated at a high temperature, e. g. substantially above 1000 C. to produce substantially uniform-sized grains or crystals. The precise size of the grains is determined by the temperature at which the treatment is effected and this will be dictated by the purpose for which the resulting powder is to be used. For instance, we have found that treatment at a Itemperature between 1100 C. and 1150 C. produces grains having a diameter between 0.003 and 0.005 inch.

After this heat treatment the bars are allowed to cool down to more or less room temperature and are then subsequently heated to a temperature between substantially 500 C. and substantially 800 C. for a period of from one to forty hours, the exact time and temperature being determined by the composition of the steel. In the I case of an 18-8 type stainless steel with a carbon content of 0.17% a temperature of 650. C. and a period of three hours was found to be satisfactory.

During this second heat treatment the carbon in the steel diifuses with moderate rapidity, but the chromium diffused only very slowly. Chromium' carbide is thus formed, mostly at the grain boundaries, the carbon being drawn from the whole mass of the steel and the chromium being drawn mainly from the outer portions of the. grains. There is thus produced a chromiumdepleted layer of material adjacent to each grain boundary and the corrosion resistance of the chromium-depleted layer is less than that of the remainder of the grain.

The bars so treated are then either pickled under normal pickling conditions by any standard well-known method or, alternatively, electrolytically pickled, in order to, remove scale or other foreign matter which may be adhering to the surface of the bars, and which would contaminate the powder ultimately'produced.

After pickling, the bars are found to be in a bright and clean condition.

Two of the bars are then immersed as electrodes in an electrolyte, suitable electrolytes being givenhereafter, which is maintained at more or less room temperature and an electric current is passed between the electrodes. Each bar of steel is made alternately anode and cathode by arranging for reversal in the direction of flow. of the electric current. This current reversal may have a period of between substantially ten seconds and substantially five minutes. The current density will be determinedby ,the particular electrolyte being used, but will be between from about 0.01 to 20 amperes for each square inch of the electrode area Which is immersed. For instance,'with electrolyte A, referred to hereinafter, the .current density is suitably 0.48 ampere per square inchand with electrolyte G referred to hereinafter, a suitable current density of 0.53 ampere per square inch.

Reversal of the direction of flow of the electric current through the electrolyte causes rapid'dissolution of the stainless steel electrodes to take place at the grain boundaries. Eventually each stainless steel bar is such that the grains are more or less surrounded by a corroded area and the metal begins to disintegrate with stainless steel grains falling to the bottom of the cell within which the electrolyte is contained. v

The stainless steel grains which fall to the bottom of the cell may be removed continuously or by intermittent stoppage of the process. The recovered grains are freed from heavy metal contamination, if necessary, by treatment with nitric acid and are freed from electrolyte and/or nitric acid by washing with water, after which they are dried. It isfound that the powder so produced has substantially the same chemical composition as the original stainless steel, it has good flow properties and is suitable for powder metallurgy applications.

By a suitable choice of conditions, the yield of powder may be made'more than equivalent to the quantity of electricity passed through the solution since the electric current is being used-merely to cause intergranular corrosion and not to cause electrolysis as such. At the-same time, though some of the starting material may be lost by solution into the electrolyte, any loss will tend to be minimised in view of such suitable choice of conditions.

It will be appreciated that as opposed to having an anode and a cathode as described above, a plurality of anodes and a plurality of cathodes may be employed, although this may necessitate the use of higher current. If desired, the plurality of stainless steel cathodes may be replaced by a lead sheet or bar, the bars forming the anodes being then arranged in pairs. The bars of a pair are, during passage of the electric current through the bath, connected alternately to the source of electrical Patented May 1, 1956 energy soflhatonly one bar of each pair is connected as anode for a given period, the other bar of the pair at that time being isolated electrically from the source of electrical energy. Such period may usefully be between substantially five seconds and ten minutes andat the end of 5 this period the other bar of the pair will automatically be connected as anode instead of being isolated electrically whilst the first-mentioned bar of the pair which was formerly connected as anode will be disconnected from the source of electrical energy. The other bar will remain connected as anode for the same period during which the first-mentioned bar was connected as anode, after which the first-mentioned bar will again be connected asanode. With such an arrangement, the electrolyte may be maintained at room temperature by making the lead cathode hollow and allowing cold water to circulate through it so as to absorb heat from the electrolyte.

Electrolytes which have been found to be suitable for this'disintegration process include the following:

A 13 grams of copper sulphate crystals.

53 cent sulphuric acid (S. G.'1.84) per litre. 111 grams ofcopper sulphate crystals.

53 cc. of sulphuric acid (S. G. 1.84) per litre. 130 grams of copper sulphate crystals per litre. 8.85 grams of copper chloride crystals.

91 cc. of hydrochloric acid (S. G. 1.16) per litre. 17.7 grams of copper chloride crystals.

93 cc. of hydrochloric acid (S. G. 1.16) per litre. 100 cc. of sulphuric acid (S. G. 1.84) 80 cc. of hydrofluoric acid (commercial) per litre. 212 cc. of nitric acid (S. G. 1.2)

39 cc. of hydrofluoric acid (commercial) per litre. 15.4 grams of mercuric sulphate crystals.

53 cc. of sulphuric acid (S. G. 1.84) per litre. 53 cc. of sulphuric acid (S. G. 1.84) 1.25 grams of potassium dichromate crystals per litre. 53 cc. of sulphuric acid (S. G. 1.84) 1.3 grams of copper sulphate crystals. 1.5 grams of hydrogen peroxide per litre. 100 cc. of sulphuric acid (S. G. 1.84)

1.5 grams of hydrogen peroxide per litre. 45 cc. of orthophosphoric acid (S. G. 1.75) 3 grams of hydrogen peroxide. 0.019 gram of copper sulphate crystals 0.3 gram of glue per litre.

All these solutions with the addition of hydrogen peroxide, glue, or copper sulphate crystals, where they are not already included, have also been found to be suitable. Furthermore, the following additions may be made to the 60 specific solutions:

l lQF WUQ 4; A. per cent of ethyl alcohol, or

3 per cent of glue solution. C. From 1 to 10 per cent of sulphuric acid. F. 6 per cent of copper sulphate crystals or 40 per cent of ethyl alcohol.

In obtaining stainless steel powder for powder metal lurgical processes in accordance with the invention it is necessary, or at least desirable, to balance the carbon content between the conflicting claims of eflicient powder production (intergranular attack being facilitated by a high carbon content) and the need for a reasonablylow carbon content (so as to guard against gross embrittlement in a final sintered product).

Consequently, the carbon content should lie between the limits 0.1% and 0.27%. Preferably, but not necessarily, the carbon content should lie between 0.15% and 0.19%.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

The method of producing stainless steel powder, comprising heating stainless steel in massive form and having a carbon content of between 0.1% and 0.27% to a temperature between 500 C. and 800 C. for a period of from one hour to fortyhours to form carbides on the grain boundaries thereof, immersing said steel so treated in an electrolytic bath in which said treated steel is at least one of the electrodes in said bath, and passing an electric current between the electrodes in said bath and periodically reversing the direction of flow of said current to make said heat treated steel electrode alternately anode and cathode and thereby induce electrolytic iutergranular attack in said steel electrode and dissolution of said heat treated steel and precipitation of the grains thereof.

References Cited inthe file of this patent UNITED STATES PATENTS Blaut et al. Apr. 26, 1938 Wulfi Sept. 17, 1946 OTHER REFERENCES 

